Project Summary Autism spectrum disorder (ASD) affects approximately 1 in 59 children in the United States1 and incurs substantial public health burden over the lifespan2. Although one of the strongest predictors of later independence for individuals with ASD is early language ability3,4, the mechanisms underlying disrupted language development in ASD are not well understood. Recent research indicates that the transition from crawling to walking represents a sensitive period for language development, during which infants with ASD exhibit atypical trajectories of language growth. For typically-developing (TD) infants, walk onset is closely linked with a sudden increase in infants? rate of language acquisition5?8 and dramatic changes in how infants perceive and interact with their environment. Emerging evidence indicates that infants with ASD do not exhibit the same ?spurt? in language development following walk onset as their TD peers5, but reasons for this difference are not yet known. We propose that gait control, the ability to effectively regulate and coordinate the rhythmic movements needed for walking, is a fundamental mechanism supporting language growth following walk onset that is impaired in infants with ASD. Poor gait control may impede language acquisition after walk onset by making it difficult for children to access the increased communicative learning opportunities that walking affords, including seeing more faces9 and having hands free to gesture10 and share objects with others11,12. Regions of the brain known to support gait and postural control exhibit atypical structure and function in individuals with ASD13,14, and older children and adults with ASD exhibit poorer gait control15?17, indicating that gait control may be fundamentally disrupted in ASD. However, no studies to date have prospectively assessed early gait control in infants with ASD. The goal of the proposed research is to investigate gait control as a potential mechanism linking ASD to change in the rate of language growth after walk onset. This study will test whether gait control 4 weeks after walk onset (measured by a pressure- sensitive gait mat) predicts change in the rate of language growth following walk onset in infants at high and low familial risk for ASD (Aim 1). Next, we will examine whether ASD symptoms at 24 months are associated with gait control after walk onset (Aim 2). Finally, the proposed study will also explore whether innovative computer vision technology can provide an infant gait assessment tool that is more scalable to settings outside the lab than current gold-standard pressure-sensitive gait mats (Aim 3). This study will be the first to explicitly test the relationships between gait control, language development, and ASD symptoms. In so doing, it will yield a deeper understanding of the link between early motor and language development during this sensitive period, with the potential to identify possible targets for early intervention to promote improved language outcomes in infants with ASD. Taken together, this project and the accompanying training plan will prepare the applicant for a research career investigating the early motor and social communication development in ASD.